1. Technical Field of the Invention
The present invention relates to a dielectric filter, a duplexer, and a communication system.
2. Description of the Related Art
A dielectric filter including a plurality of dielectric resonators in a single dielectric block, a dielectric filter shown in FIG. 21, for example, has been known. In this dielectric filter, two resonator holes 2a, 2b are provided through the end surfaces 1a, 1b facing each other of a dielectric block 1. Nearly all over the outside surface of the dielectric block, an outer conductor 5 is provided. A pair of input and output electrodes 6, 6 are provided on the outside surface of the dielectric block 1 in such a way that the electrodes are kept a fixed distance away from the outer conductor 5 and are not conductive to the outer conductor 5. An inner conductor 7 is provided on subatantially all of the inside surface of the resonator holes 2a, 2b and a gap 8 is provided between the inner conductor 7 and the outer conductor 5 extended into the inside surface on this side of the resonator holes 2a, 2b.
In a conventional dielectric filter, changing the distance between the axes of the resonator holes 2a, 2b adjacent to each other or changing the external dimensions of their dielectric block was required to adjust the degree of electromagnetic coupling between the resonator holes 2a and 2b. This fact has brought about the following problems. Dies of various dimensions were needed to prepare for manufacture of dielectric blocks and adjustment of the degree of electromagnetic coupling between dielectric resonators was complicated. As a result, not only there was the lack of flexibility in changing their design, but also manufacturing cost of dielectric filters became high.